Curable coating compositions have wide application in number of fields, as they are useful for protecting various kinds of surfaces from severe chemical and physical environments. A baking enamel, for example, may be applied to an object and then heated to form a tough, chemical-and-light resistant coating. Such a material needs to have at least one form of cure such as UV or thermal cure. A coating for an object that does not have a smooth surface, such as a conformal coating, may profit from use of dual cure methods, such as UV cure step followed by a thermal cure step. The UV cure step can provide a quick cure so that the coating is dry to the touch and the object can be handled, and the thermal cure can then proceed to harden the coating in shadowed areas.
A more elaborate process is needed where it is desirable to coat only part of an object's surface. A solder mask, for example, is a permanent coating for a printed circuit board that must not cover certain parts of the circuitry on the board such as the contact points. A UV-curable solder mask is applied as a coating to the entire board, and then exposed to ultraviolet light through a pattern, or image. Where the light falls on the coating, the coating hardens, or cures. The portions that are not exposed to the light remain unhardened and are washed away in a developer solution. Sometimes it is desirable to further treat the solder mask to toughen it. In that case, the coating might be baked to further improve its chemical, heat, and moisture resistance.
The situation may be even more complicated where a temporary, partial coating is desired. A primary photoresist, for example, is a temporary coating used in a photographic-style process to prepare the circuitry on a printed circuit board. The primary photoresist is applied as a coating to an object which is a substrate laminated with a layer of copper. The coated object is then exposed to ultraviolet light through a pattern, or image. The exposed portion of the coating hardens, or cures in response to the light, and becomes resistant to a developer solution. The unexposed portions remain unhardened and are washed away by the developer solution, thereby uncovering portions of the copper layer. The unprotected portions of the copper layer are then removed in an etch bath, leaving the substrate uncovered in some areas, and covered by a layer of copper-plus-resist in others. Next, the resist layer is stripped from the copper, leaving the substrate with a copper circuit pattern on it. The various steps of imaging, developing, and etching must be exquisitely precise to meet the ever-increasing demands for fine, error-free lines.
Materials which are useful in the above-described processes are well known, and are available as formulations that contain and are developable in solvents made of volatile organic compounds (VOCs). VOCs, however, have been identified as pollutants and have been targeted by a variety of regulations aimed at reducing their use. Accordingly, efforts have been directed toward obtaining coating materials that can be formulated in water (or, are waterborne) and are water-developable.
One technique for rendering a coating material water developable is to add acidic functional groups to an organic molecule that is otherwise known to yield good physical properties when formulated as a coating. For example, certain commercially available formulations use polymers modified to incorporate significant amounts of carboxylic acid functional groups in order to confer water dispersibility on an otherwise hydrophobic molecule. Such coatings are typically developable in alkaline water (1% carbonate) solutions. Once the coating is applied, imaged and developed, however, any acid groups remaining in the coating need to be destroyed or treated to improve the coating's chemical and humidity resistance as well as to reduce corrosion and ionic permeability for better electrical insulation properties. In addition, since the coatings are developed with alkaline water, the used developer solution needs to be neutralized and treated before disposal, adding to the environmental processing load. Further, these formulations also typically include additives with epoxy functional groups in order to allow thermal curability. Epoxy groups and carboxylic acid groups react with one another. As a result, the epoxy and the acid portions of the formulation have to be mixed and used within a limited period of time, on the order of hours to, at most, several days.
A coating system that is truly developable in water as opposed to alkaline water, which relies on little or no VOC content in its formulation, and which can be UV curable, thermally curable, or both, would be particularly valuable for use in one part, curable coatings and sealants, as well as resists and solder masks.
The inventor has found that such a coating system can be made using oligomers having acrylate and/or latent acrylate unsaturation. Such oligomers have labile hydrophillic groups and can be reversibly converted from water soluble to water insoluble forms. The stable, water soluble form is converted under mild heat to a water insoluble form that is photo- or heat- curable.